The present invention relates to an improved multi-beam electron gun and particularly to an electron gun in which a screen grid (G2) electrode is individually attached to a common ceramic member and positioned relative to a control grid (G1) electrode.
U.S. Pat. No. 4,298,818 issued to McCandless on Nov. 3, 1981 discloses an electron gun having a plurality of cathode assemblies and at least two spaced, successive electrodes having aligned apertures which are individually attached directly to a metallized pattern on a common ceramic member. Longitudinal spacing between the successive electrodes is provided by the accurately dimensioned parallel flanges on each of the electrodes and by the flatness of the surface of the ceramic member to which the electrodes are attached. Variations in surface flatness of the ceramic member or in the flange heights of the electrodes will cause corresponding variations in spacing between the successive electrodes. Proper operation of a multibeam cathode-ray tube utilizing such an electron gun requires that the spacing and alignment between successive electrodes in the electron gun be accurately maintained. Apertures that are misaligned by as little as 0.127 mm (0.5 mils) can cause distorted beam shapes and degrade the performance of the tube.
U.S. Pat. No. 4,500,808, issued to McCandless on Feb. 19, 1985, describes an improved electron gun similar to that of U.S. Pat. No. 4,298,818 except that the second electrode comprises a composite structure having a metal support plate brazed directly to a metallized pattern on one surface of a certamic support. The metal support plate has a window therein opposite each of the apertures in a first electrode which is also brazed directly to a separate metallized pattern on the same surface of the ceramic support. Separate metal plates are brazed to the metal support plate and close the windows therein. Each of the metal plates has a single electron beam-defining aperture therein which is separately aligned with one of the apertures in the first electrode. This structure provides more accurate alignment of successive grid apertures than previous structures; however, the longitudinal spacing between the successive electrode also depends upon the flatness of the surface of the ceramic member and the flange heights of the first electrode and of the metal support plate of the second electrode.